The invention relates to a process of preparing small crystal size metal titanates. In particular, the invention relates to the preparation of small crystal size alkali earth titanates.
Metal titanates are used in various electronic applications due to their electrical, electro-optical and electro-mechanical properties, among others. Known compounds of this kind include, e.g. barium titanates and barium strontium titanates, i.e., BSTO materials. They are used in multilayer ceramic capacitors, dynamic access random memories (DRAM), positive temperature coefficient of resistance thermistors, sensor applications, piezoelectric equipment and as ceramic insulators, in general, or in corresponding applications. Their use as insulating materials is based on a high dielectric constant.
Numerous processes in preparing titanates are known but, typically, they employ processes that require high temperatures or pressures, to achieve the desired end product. Many of the published processes also deal with the recovery of titanates under very dilute solution conditions, which is generally not desirable on an industrial scale. When the concentrations are high, a greater production capacity is achieved and less energy is required for possible evaporations and other factors that affect the economic efficiency of the processes. Furthermore, both the handling properties and the costs of the raw materials should be such that the materials would be applicable in the industrial environment of use. In so far as the preparation begins with crystalline titanium dioxide, TiO2, then typically, there would be a problem of running other different size metal ions and/or those with a different charge into the original crystal lattice. A change in the crystal lattice does not take place under all conditions, at least not under usable conditions. It is generally known that in the preparation of titanates, a relatively high temperature, of as much as over 1000° C., must be used, or in solution reactions, a special pressure reactor must be used, where the temperature can be raised to a high level. Furthermore, many reactors employ special gaseous atmospheric conditions.
In literature, it is known that nanocrystalline barium titanate (BaTiO3) is a usable material in various commercial applications, due to its electric and ferroelectric properties, in particular. In MLCs (multilayer capacitors), the use of barium titanate is based on its high dielectric constant, and in converters and sensors, on its piezoelectric properties. Other applications include electro-optical devices and various chemical switches.
The ferroelectric properties and the use of barium titanate in applications are based on the crystal structure of the compound. The crystal structure is cubic at high temperatures and barium titanate does not have ferro-electric properties at that time. Along with a decrease in temperature, the cubic structure distorts into tetragonal, orthorhombic and rhombohedral structures that are ferro-electric. Additionally, BaTiO3 can also have a hexagonal structure.
The problems encountered with the preparation of barium titanate have included variations in the crystal structure, a non-stoichiometric composition, a poor repeatability of the electric properties and the preparation of sufficiently small crystals, e.g. below 200 nm. This has resulted in a development of new processes of preparation that take place in a solution. The preparation processes of the industrial scale favour low reaction temperatures and short reaction times. Such preparation processes include the sol-gel process, homogeneous precipitation, hydrolysis and hydrothermal process, as well as a process based on the condensation of vapours. Furthermore, barium titanate can be prepared by means of an ultra sound pyrolysis or by degrading organometallic compounds. Of these processes, the most suitable ones for the industrial scale include the traditional annealing preparation or the hydrothermal process.
In a solid-state reaction, barium carbonate and titanium dioxide are heated at high temperatures, such as 800-1430° C. Another disadvantage is the strong sintering of crystals and the remaining impurities.
In the hydrothermal process, barium titanate is hydrothermally formed in alkaline (pH>12) aqueous solutions at a raised temperature and pressure. The required temperatures are 100-300° C., when the pressure is within 0.5-5 MPa. As starting materials, for example, barium hydroxide and barium chloride can be used and, as sources of titanium, titanium alkoxides, titanium oxides or titanium oxide gels can be used. The dynamic interactions between the TiO2 molecules and Ba2+ and OH− ions determine the generation of the nucleating centres of barium titanate and the mechanism of crystal growth. Typically, the hydrothermal reaction includes a solution drifting through slurry, its adsorption into the surface, and the dehydration, surface diffusion of the components of the solution and, finally, the formation and growth of the crystal. The tetragonal form can be prepared hydrothermally, when the preparation temperature is within 450-600° C., or when chloride ions are added to the reaction mixture. The tetragonal structure can also be formed by heating a product that has been prepared hydrothermally at about 1000° C.
One problem with the hydrothermal process is its unsuitability to an economic industrial preparation process, as the pressure reactor causes safety risks and incurs investment costs. Furthermore, the high pressure and temperature considerably slow down the efficiency of the batch process, due to the longer heating and cooling stages required by the same, thus essentially decreasing the energy effectiveness of the process.
In addition, using a water-based process may create side products, such as BaCO3, and unwanted crystal forms, such as Ba2TiO4, which must be removed from the product. When using precipitation with NaOH or KOH, alkali metal ions may remain in the crystal structure; therefore, the preparation of a pure nanocrystalline BaTiO3 crystal form has proven to be challenging.
In the Patent Specification EP141551, barium titanate, strontium titanate or a powder that consists of the solid solution thereof is prepared, its average particle size with a regular spherical shape being 0.07-0.5 μm, BET 20 m2/g or less, and the crystal size 0.05 μm or larger. In the process, titanium oxide hydrate is allowed to react with barium hydroxide and/or strontium hydroxide at a temperature of 60-110° C., so that 120-2000 mol of water per mol of titanium is present in the reaction. Titanium oxide hydrate is preferably selected from among orthotitanium acid, metatitanium acid and titanium oxide, of which orthotitanium acid is the most preferable due to its high reactivity. The reaction time is preferably 30 min or longer, in order for the reaction to proceed as far as possible. If the reaction temperature is below 60° C., it was observed that the reaction speed was too slow for a practical implementation. On the basis of examples, the reaction temperature is preferably about 100° C. The combined mole amount of barium hydroxide and/or strontium hydroxide used in the process, in relation to the amount of titanium oxide hydrate, is 1.3-5.0. The high-temperature requirement is a consequence of the titanium starting materials used in the process and the apparently weak reaction conditions, which make an effective reaction between the starting materials difficult. Furthermore, the dilute titanium content and the CO2 limitations, as well as the use of a nitrogen treatment, render the process problematic on an industrial scale.
The Patent Specification US200410028601 describes a process of preparing powder that has the ABO3 perovskite structure. In this process, A-hydroxide, such as Ba(OH)2.8H2O, is dissolved in water to form a strong solution or melt, and B-oxide, such as anatase TiO2, is added to this hydroxide, whereupon they are allowed to react with each other at a temperature of about 70° C., based on the examples. In that case, extremely small, microcrystalline particles are formed, such as cubic BaTiO3 crystals, which contain discernible impurities. The BET of the particles is about 60-100 m2/g and the crystal size about 20 nm. The product is dried and calcined at a temperature of over 900° C., whereby tetragonal BaTiO3 is obtained, its BET being below 10 m2/g and its particle size over 100 nm.
The Patent Specification WO2007015622 describes a hydrothermal synthesis for the preparation of barium titanate with a particle size of below 1000 nm. In the process, the aqueous titanium acid compound, obtained from a sulphate process, is reacted with crystalline titanium oxide and barium hydroxide at 60-300° C. and at a high pressure of 5-50 Kgf/cm2 for 10 minutes −10 hours, using a barium hydroxide excess. The product is dried and calcined under reducing conditions at 600-1400° C. The Ba/Ti molar ratio of the product is 1.000±0.002. The reducing condition and the high pressure that are used in the process make it impractical, even dangerous, for industrial processes.
The Patent Specification U.S. Pat. No. 5,445,806 discloses a process of preparing a perovskite type of product, such as tetragonal barium titanate, its average particle size being below 300 nm. In the process, an alkali earth compound, such as Ba (and/or Sr) are reacted, for example, with a Ti4+ compound at a temperature of about 100° C. and at an atom ratio of Ba(and/or Sr)/Ti of 1-1.4. The barium compound can be a hydroxide and titanium compound, e.g. a titanium oxide hydrate prepared from titanium tetrachloride, preferably an organometallic compound. The dried product thus obtained is preferably calcined at a temperature of 1000-1100° C. and purified by removing the extra Ba. The product obtained is BaTiO3, wherein the Ba/Ti ratio is 1-1.4 and the average particle size is 50-300 nm. The chlorine residual, resulting from the chloride starting materials, may weaken the quality of the end product and the product must be calcined at a high temperature to provide the desired titanate product.
The Patent Specification U.S. Pat. No. 6,352,681 describes a process of preparing barium titanate, wherein the aqueous solution of a titanium compound, such as water-soluble titanium salt, preferably halide, and the aqueous solution of a barium compound, such as barium hydroxide, and alkali metal hydroxide are added to an alkaline solution and kept at 90° C. whilst being subject to stirring. The barium titanate thus generated is recovered and dried. In the reaction, the molar ratio of the titanium compound and the barium compound was kept within 0.8-1.2. The product thus obtained is micro-crystalline; its particle size is below 60 nm and its Ba/Ti atom ratio is close to one.
The Patent Specification U.S. Pat. No. 4,859,448 describes a process of preparing powdery barium titanate from titanium dioxide and barium hydroxide, Ba(OH)2.8H2O. The titanium dioxide is amorphous titanium dioxide that is prepared from titanium alkoxide, such as titanium tetraethoxide, its water content being about 0.5 mol. The compounds are reacted in water at a temperature of 60-95° C., so that there is no carbon dioxide present in the reaction conditions. After the reaction, the non-reacted barium hydroxide is removed and the product is dried. The product obtained is tetragonal BaTiO3, the particle size of which is 10-500 nm. The starting materials used in the process cannot be considered on the industrial scale and the process described is too sensitive to external disturbances, such as variations in the carbon dioxide content of the environment.
The purpose of the present invention is to disclose an industrially simple and effective process of preparing small crystal size metal titanates that have a high dielectric constant.
Another purpose of the present invention is to provide a low-temperature process of preparing metal titanates, Ba and Sr or BaSr titanates, in particular.